ABSTRACT: The goal of this Program Project is understanding the sequence organization of the eukaryotic genome. It appears evident that the structural basis of gene regulation in higher organisms lies in the specific organization of genomic DNA sequences. Since cancer is clearly a disease in which the basic patterns of gene regulation go awry, knowledge of DNA sequence organization is of the greatest importance to the basic problems of cancer biology. Methods to be used to study sequence organization include nucleic acid reassociation; electron microscope visualization of nucleic acid duplexes; single-and double-strand specific enzyme digestion; and other general approaches in the chemistry of nucleic acid interactions such as measurement of hyperchromicity, hydroxyapatite, chromatography, measurement of the kinetics of reassociation, and DNA fragment size determinations. Specific projects include development of techniques for ferritin labeling of short DNA fragments; electron microscope observations of reassociation products formed from sized fragments and from reaction of short repetitive sequence elements with long DNA; studies of the sequence content and significance of folded circles of Xenopus DNA and other DNAs; isolation of hemoglobin genes and analysis of their sequence environment; mapping of the tRNA genes on Drosophila DNA; analysis of repetitive sequence organization in the genomes of Drosophila, rat and also human; study of repetitive sequence content and length by use of single- and double-strand specific enzymes; studies on the degree of sequence homology in nonrepetitive and repetitive sequence classes; studies on the clustered repetitive DNA; and evolutionary comparisons of interspersion patterns and sequence homology among related organisms.